1. Field of the Invention
The present invention relates to an austenitic, substantially ferrite-free steel alloy and the use thereof. The invention also relates to a method for producing austenitic, substantially ferrite-free components, in particular drill rods for oilfield technology.
2. Discussion of Background Information
When sinking drill holes, e.g., in oilfield technology, it is necessary to establish a drill hole path as exactly as possible. This is usually done by determining the position of the drill head with the aid of magnetic field probes in which the earth's magnetic field is utilized for measuring. Parts of drill rigs, in particular drill rods, are therefore made of non-magnetic alloys. In this connection, a relative magnetic permeability μr of less than 1.01 is required today, at least for those parts of drilling strings that are located in the direct vicinity of magnetic field probes.
Austenitic alloys can be substantially ferrite-free, i.e., with a relative magnetic permeability μr of less than about 1.01. Austenitic alloys can thus meet the above requirement and therefore be used in principle for drilling string components.
In order to be suitable for use in the form of drilling string components, in particular for deep-hole drillings, it is further necessary for an austenitic material to exhibit minimal values of certain mechanical properties, in particular of the 0.2% yield strength and the tensile strength, and to be able to withstand the dynamically varying stresses that occur during a drilling operation, in addition to having a high fatigue strength under reversed stresses. Otherwise, e.g., drill rods made of corresponding alloys cannot withstand the high tensile and pressure stresses and torsional stresses that occur during use or can withstand them only for a short time in use; undesirably rapid or premature material failure is the result.
As a rule, austenitic materials for drilling string components are highly alloyed with nitrogen in order to achieve high values of the yield strength and tensile strength of components such as drill rods. However, one requirement to be taken into consideration is a freedom from porosity of the material used, which freedom from porosity can be influenced by the alloy composition and production method.
In this regard, economically favorable alloys naturally are alloys which upon solidification under atmospheric pressure result in pore-free semi-finished products. However, in practice, such austenitic alloys are rather rare because of the high nitrogen content, and in order to achieve a freedom from porosity a solidification under increased pressure is consistently necessary. A melting and solidification under nitrogen pressure can also be necessary in order to incorporate sufficient nitrogen in the solidified material, if otherwise there is an insufficient nitrogen solubility.
Finally, austenitic alloys that are provided for use as components of drilling strings should have a good resistance to different types of corrosion. In particular a high resistance to pitting corrosion and stress corrosion cracking is desirable, above all in chloride-containing media.
According to the prior art, austenitic alloys are known which each meet some of these requirements, namely being substantial ferrite-free, having good mechanical properties, being free of pores and exhibiting a high corrosion resistance.
Articles made of a hot-worked and cold-worked austenitic material with (in % by weight) max. 0.12% of carbon, 0.20% to 1.00% of silicon, 17.5% to 20.0% of manganese, max. 0.05% of phosphorus, max. 0.015% of sulfur, 17.0% to 20.0% of chromium, max. 5% of molybdenum, max. 3.0% of nickel, 0.8% to 1.2% of nitrogen which material is subsequently aged at temperatures of above 300° C. are known from DE 39 40 438 C1. However, as noted by some of the same inventors in DE 196 07 828 A1, these articles have modest fatigue strength under reversed stresses of at best 375 MPa, which fatigue strength is much lower still in an aggressive environment, e.g., in saline solution.
Another austenitic alloy is known from DE 196 07 828 A1, mentioned above. According to this document, articles are proposed for the offshore industry which are made of an austenitic alloy with (in % by weight) 0.1% of carbon, 8% to 15% of manganese, 13% to 18% of chromium, 2.5% to 6% of molybdenum, 0% to 5% of nickel and 0.55% to 1.1% of nitrogen. Such articles are reported to have high mechanical characteristics and a higher fatigue strength under reversed stresses than articles according to DE 39 40 438 C1. However, one disadvantage thereof is a low nitrogen solubility that is attributable to the alloy composition, which is why melting and solidification have to be carried out under pressure, or still more burdensome powder metallurgical production methods have to be used.
An austenitic alloy which results in articles with low magnetic permeability and good mechanical properties with melting at atmospheric pressure is described in AT 407 882B. Such an alloy has in particular a high 0.2% yield strength, a high tensile strength and a high fatigue strength under reversed stresses. Alloys according to AT 407 882 B are expediently hot worked and subjected to a second forming at temperatures of 350° C. to approx. 600° C. The alloys are suitable for the production of drill rods which also adequately take into account the high demands with respect to static and dynamic loading capacity over long operating periods within the scope of drill use in oilfield technology.
Nevertheless, as was ascertained, material failure can occur because during use drilling string components such as drill rods are subjected to highly corrosive media at high temperatures and additionally are subjected to high mechanical stresses. Consequently, stress corrosion cracking can occur. Since drill rods and other parts of drill installations may also be in contact with corrosive media during down time, pitting corrosion can likewise contribute substantially to material failure. In practice, both types of corrosion cause a shortening of the maximum theoretical working life or operational time of drill rods that one would expect based on the mechanical properties or characteristics.
The known alloys discussed above show that highly nitrogenous austenitic alloys which can be melted under atmospheric pressure to form at least substantially pore-free ingots do not meet the requirements of good mechanical properties and at the same time high resistance to corrosion during tensile and compressive stress and high resistance to pitting corrosion in a satisfactory manner.
It would be advantageous to have available an austenitic steel alloy which can be melted at atmospheric pressure and processed to form pore-free semi-finished products and which at the same time has a high resistance to stress-corrosion cracking and to pitting corrosion with good mechanical properties, in particular with a high 0.2% yield strength, a high tensile strength and a high fatigue strength under reversed stresses. It would also be advantageous to have available an austenitic, substantially ferrite-free alloy.